Sensor Plane Research Articles

The ‘best-correlated pixels’ method for solid mass flow measurements usingelectrical capacitance tomography

A novel method of calculating a solid velocity field based on a twin-planeelectrical capacitance system is presented. This method introduces the concept ofthe best-correlated pair of pixels instead of the classical assumption that a flowpattern between two sensor planes is frozen. Preliminary results and futureapplications of the proposed method are discussed.

A silicon strip detector used as a high rate focal plane sensor for electrons in a magnetic spectrometer

Abstract A silicon strip detector was developed as a focal plane sensor for a 300 MeV electron spectrometer and operated in a high rate environment. The detector with 500 μm pitch provided good position resolution for electrons crossing the focal plane of the magnetic spectrometer system which was mounted in Hall C of the Thomas Jefferson National Accelerator Facility. The design of the silicon strip detector and the performance under high counting rate ( ⩽2.0×10 8 s −1 for ∼1000 SSD channels) and high dose are discussed.

Target localization in the three-dimensional space by wavelength multiplexing

A method to localize a target in the three-dimensional space is presented. Each different position of the target on the depth axis produces, when captured with a CCD camera, an image of a different size on its sensor plane. The size of this image depends only on the distance between the target and the camera. The use of a white light optical correlator that gives us a different response depending on the scale of the input image permits us to know the depth position of the particular target. The obtained results demonstrate the utility of the newly proposed method.

Optimal dithering of focal plane arrays in passive millimeter-wave imaging

Passive millimeter-wave (PMMW) imagery has tremendous potential for imaging in adverse conditions. However, poor resolution and long acquisition times pose serious limitations to this potential. Therefore, an important issue is the optimization of the sampling pattern. Ordinarily, a focal plane sensor array has sensors placed in a rectangular grid pattern at subNyquist density, and the array must be dithered to sample the image plane at the Nyquist density in each dimension. However, the Nyquist density oversamples the image due to the usually circular support of the diffraction-limited image spectrum. We develop an efficient algorithm for optimizing the dithering pattern so that the image can be reconstructed as reliably as possible from a periodic nonuniform set of samples, which can be obtained from a dithered rectangular-grid array. Taking into account the circular frequency support of the image, we sequentially eliminate the least informative array recursively until the minimal number of arrays remain. The resulting algorithm can be used as a tool in exploring the optimal image acquisition strategy.

A New CMOS Hall Angular Position Sensor (Neuer CMOS-Hall-Winkelpositionssensor)

The new angular position sensor consists of a combination of a permanent magnet attached to a shaft and of a two-axis magnetic sensor. The permanent magnet produces a magnetic field parallel with the magnetic sensor plane. As the shaft rotates, the magnetic field also rotates. The magnetic sensor is an integrated combination of a CMOS Hall integrated circuit and a thin ferromagnetic disk. The CMOS part of the system contains two or more conventional Hall devices positioned under the periphery of the disk. The ferromagnetic disk converts locally a magnetic field parallel with the chip surface into a field perpendicular to the chip surface. Therefore, a conventional Hall element can detect an external magnetic field parallel with the chip surface. As the direction of the external magnetic field rotates in the chip plane, the output voltage of the Hall element varies as the cosine of the rotation angle. By placing the Hall elements at the appropriate places under the disk periphery, we may obtain the cosine signals shifted by 90°, 120°, or by any other angle.

Magnetic field sensors from polycrystalline manganites

We report on the fabrication of magnetic sensors based on bulk La 0.67Sr 0.33MnO 3 and La 0.67Ba 0.33MnO 3 material. The sensors were characterized at fields of 0–8 T and temperatures of 4.2–300 K. The sensors display a maximum sensitivity of ∼200%/T at room temperature within a field range of 1–3 mT. For the sensor geometry investigated here, the low-field magnetoresistance (MR) of the sensors measured in three field orientations with respect to the sensor plane is strongly anisotropic. The high-field MR, in contrast, is found to be field-orientation independent. Periodic response of the sensor’s resistance to the angle between the field and sensor-plane is demonstrated at room temperature.

Development of high spectral resolution CdZnTe pixel detectors for astronomical hard X-ray telescopes

Cadmium zinc telluride detectors (CdZnTe) offer the possibility of achieving excellent spectral and spatial resolution for compact hard X-ray sensors operated without cryogenics. These characteristics make these detectors ideal for several astrophysical applications, including focal plane sensors for multilayer hard X-ray telescopes, and in the image plane of coded-aperture imagers. We are developing a CdZnTe pixel detector and low-noise VLSI readout for use at the focal plane of the balloon-borne High Energy Focusing Telescope (HEFT). In particular, we have concentrated on achieving excellent spectral resolution in a device with 600 μm pixels. Here we report results from the testing of our fully functional prototype hybrid detector. The detector (an 8×7×2 mm 3 single crystal of CdZnTe with an 8×8 pattern of platinum contacts on one side and a monolithic contact on the other) is indium bump-bonded to our 64-channel readout chip, which contains a charge-sensitive preamplifier, shaping amplifier, and combination peak stretcher/discriminator for each pixel. In this paper, we present results taken at room temperature, demonstrating a low-energy threshold near 5 keV and electronic noise of 1.5 keV FWHM at 60 keV. Subsequent testing of this detector at −10°C, reported in Cook et al. [Proc. SPIE 3769 (1999) 92], achieved 670 eV FWHM resolution at 60 keV, with electronic noise of 340 eV.

SAR ATR performance using a conditionally Gaussian model

A family of conditionally Gaussian signal models for synthetic aperture radar (SAR) imagery is presented, extending a related class of models developed for high resolution radar range profiles. This signal model is robust with respect to the variations of the complex-valued radar signals due to the coherent combination of returns from scatterers as those scatterers move through relative distances on the order of a wavelength of the transmitted signal (target speckle). The target type and the relative orientations of the sensor, target, and ground plane parameterize the conditionally Gaussian model. Based upon this model, algorithms to jointly estimate both the target type and pose are developed. Performance results for both target pose estimation and target recognition are presented for publicly released data from the MSTAR program.

Micro-measurement using grating microscopy

In this paper, two quantitative methods to measure micro-deformation using grating microscopy are proposed, a grating diffraction method and a full-field grating phase-shift method. A compact optical transmitting microscope with white light source is reconstructed by developing a loading and recording system. For direct strain measurement, a grating diffraction method is presented. With the help of a Bertrand lens, the Fourier spectrum of the grating is formed on the CCD sensor plane with high image quality. Software for precise, fast and automatic determination of the diffraction spot centroids is developed. Local strains are thus measured with high resolution. For the deformation measurement, a phase-shifting grating microscope method with high sensitivity and spatial resolution is proposed. Phase-shifting is based on the slab refraction effect and is realized via a tilting compensator. The system possesses a high spatial resolution (1 μm), and a displacement precision of 0.1–0.03 μm. The combination of the grating diffraction method and the phase-shifting method in the same test provides simultaneous measurement of strain and displacement, thus demonstrating that the grating techniques are viable in the microscope environment.

OPTICAL SENSOR DESIGN USING NONLINEAR PROGRAMMING

Nonlinear programming is used to design a non-contact high resolution optical system capable of sensing all six degrees of freedom of an object. This system uses two wide-beam lasers to illuminate two reflective fiducial markers placed on the object's surface. Light rays reflected from these markers are incident on three position sensitive detectors, which measure the x-y coordinates of the ccntroids of the incident light spots. The mathematical model of the system, which maps object coordinates into light spot coordinates, is very complex and highly nonlinear. The purpose of the study is to find designs that optimize three objectives: linearity error, range, and sensitivity, both singly and in combination, and to gain insight into the design tradeoffs among them. The design variables include the angular orientation of the laser beams, the positions of each of the fiducial markers, and the distance between sensor and object planes. Analysis of the model shows how these variables must be changed to improve each objective. Computational results obtained using a generalized reduced gradient (GRG) algorithm show how geometric constraints on system area and component overlap limit the attainment of each objective. Tradeoff studies show the strong tradeoffs among these objectives, and indicate that the most important design variables are the orientations of the light sources. The methodology shows great promise for reducing the development time of sensors with specific requirements, and for improving sensor performance.

Techniques for identifying dust devils in Mars Pathfinder images

Image processing methods used to identify and enhance dust devil features imaged by IMP (Imager for Mars Pathfinder) are reviewed. Spectral differences, visible red minus visible blue, were used for initial dust devil searches, driven by the observation that Martian dust has high red and low blue reflectance. The Martian sky proved to be more heavily dust-laden than pre-Pathfinder predictions, based on analysis of images from the Hubble Space Telescope. As a result, these initial spectral difference methods failed to contrast dust devils with background dust haze. Imager artifacts (dust motes on the camera lens, flat-field effects caused by imperfections in the CCD, and projection onto a flat sensor plane by a convex lens) further impeded the ability to resolve subtle dust devil features. Consequently, reference images containing sky with a minimal horizon were first subtracted from each spectral filter image to remove camera artifacts and reduce the background dust haze signal. Once the sky-flat preprocessing step was completed, the red-minus-blue spectral difference scheme was attempted again. Dust devils then were successfully identified as bright plumes. False-color ratios using calibrated IMP images were found useful for visualizing dust plumes, verifying initial discoveries as vortex-like features. Enhancement of monochromatic (especially blue filter) images revealed dust devils as silhouettes against brighter background sky. Experiments with principal components transformation identified dust devils in raw uncalibrated IMP images and further showed relative movement of dust devils across the Martian surface.

Optical Grating Diffraction Method: From Strain Microscope to Strain Gauge

The grating diffraction method for direct strain measurement is reviewed. Two systems which use this method are presented. The first system is a compact strain microscope. A Leitz optical transmitting microscope with white light source is reconstructed by developing a loading and recording system. Gratings with median density of between 40 and 200 lines/mm are used. With the help of a Bertrand lens, the Fourier spectrum of the grating is formed with high image quality on the CCD sensor plane. Software is developed to precisely, quickly and automatically determine the diffraction spot centroids. The second system is a new strain sensor based on a high-frequency grating and two Position Sensor Detectors (PSDs). The grating, attached to the surface of the specimen, is illuminated by a focused laser beam, generally with a frequency of 1,200 lines/mm. The centroids of diffracted beam spots from the grating are automatically determined using two PSD sensors connected to a personal computer. The shift of diffracted beam spots due to specimen deformation is detected. Strain sensitivity of one micro-strain can be obtained, as can a 0.4 mm spatial resolution for strain measurement. The system can be used for both static and dynamic tests.

Global wavefront reconstruction from its intensity distribution in the focal plane and Shack-Hartmann sensor images

Wavefront reconstruction from its intensity distribution in the focal plane has been studied as an inverse source problem in optics.1 Since the intensity distribution in the focal plane is related to the squared modulus of the Fourier transform of a wavefront, this problem is equivalent to reconstructing a wavefront from the modulus of its Fourier transform.

Active noise control in large circular duct using an error sensors plane

Active Noise Control (ANC) of higher order modes in a circular duct is studied. To overcome the limitations of a modal approach to locate the error sensors, an alternative strategy, referred to as the error sensor plane concept, is proposed. The basic objective of the error sensor plane concept is to create a quiet cross-section in the duct so that the noise from the primary source cannot propagate over this section. To create this quiet cross-section, a network of error microphones is located in the cross-section of the duct. To determine the required number of error microphones and their location in the quiet cross-section, a model simulating a multi-channel ANC system for a circular duct is developed. Using this model, it was deduced that to achieve good control performance, the maximum distance between adjacent error sensors had to be minimized, as well as the distance between the error sensors and the duct wall. To determine the optimal location of the error sensors, the k mean algorithm is implemented. A scale model was used in a laboratory setup to verify the efficiency of the proposed strategy to control high order modes in a circular duct. The obtained results demonstrate that, for the control to be effective, the maximum distance between each error sensor, and the limit of its zone of influence should be less or equal to 1/3 of the wavelength for the frequency considered, i.e. when D max/ λ cf < 1/3.

Real-Time Dynamic Visual Tracking Using PSD Sensors and Extended Trapezoidal Motion Planning

A real-time visual servo tracking system for an industrial robot has been implemented using PSD (Position Sensitive Detector) cameras, neural networks, and an extended trapezoidal motion planning method. PSD and directly transduces the light‘s projected position on its sensor plane into an analog current and lends itself to fast real-time tracking. A neural network, after proper training, transforms the PSD sensor reading into a 3D position of the target, which is then input to an extended trapezoidal motion planning algorithm. This algorithm implements a continuous motion update strategy in response to an ever-changing sensor information from the moving target, while greatly reducing the tracking delay. This planning method is found to be very useful for sensor-based control such as moving target tracking or weld-seam tracking in which the robot needs to change its motion in real time in response to incoming sensor information. Further, for real-time usage of the neural net, a new architecture called LANN (Locally Activated Neural Network) has been developed based on the concept of CMAC input partitioning and local learning. Experimental evidence shows that an industrial robot can smoothly track a moving target of unknown motion with speeds of up to 1 m/s and with oscillation frequency up to 5 Hz.

Towards the standardization of in vitro load transfer investigations of hip prostheses

The effect of optical refraction at an interface between optically dissimilar media is modelled in order to apply the principles of three-dimensional digital image correlation (DIC) to measure deformations on submerged objects accurately. Using an analytical formulation for refraction at an interface, a non-linear solution approach is developed to perform stereo calibration. The proposed method incorporates a simplified parametric representation for the orientation and position of an interface(s), accounting explicitly for the effects of refraction at all such interfaces in the path of each stereo camera. Separating the calibration process into two parts, a modified bundle adjustment process with an updated Levenburg–Marquardt (LM) non-linear optimization algorithm is employed to determine (a) intrinsic and extrinsic stereo camera parameters without interface refraction and (b) orientation and position of each planar interface. Detailed simulations demonstrate the efficiency, accuracy, and stability of the methodology when using multiple images of a grid pattern undergoing general rigid body motion, even in the presence of Gaussian noise in the sensor plane measurements, providing a robust framework for practical implementation of the methodology for submerged object measurements.

Strain microscope with grating diffraction method

A compact microscope system for direct strain measurement is presented. It involves the grating diffraction method coupled with mi- croscopy and image-processing technique. A Leitz optical transmitting microscope with white light source was rebuilt to incorporate a loading and recording system. Gratings with median density from 40 to 200 lines/mm are used. With the help of a Bertrand lens, the Fourier spec- trum of the grating is formed on the CCD sensor plane with high image quality. Software that can precisely, quickly, and automatically determine the diffraction spot centroids has been developed. The local strain is measured with high resolution. Various ways of improving the sensitivity are suggested. © 1999 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(99)02001-2)

Method and apparatus for active noise control of high order modes in ducts

An active noise control system for effective control of higher order modes of noise propagation within a duct is disclosed. A plurality of error sensors is disposed within an error sensors plane, which plane is perpendicular to the longitudinal axis of the duct. The disclosed process and apparatus minimizes the mean square distance between the points of the area associated to each error sensor. The resulting arrangement of error sensors optimizes the overall area that the error sensors can control and consequently the global efficiency of the controlling system.

Acoustic emission surface responses from mixed-mode cracking

An analytical expression for the relationship between acoustic emission (AE) surface motion and displacement discontinuity of a finite crack extending in mixed mode was investigated based on an analogy to earthquake problems in seismology and crackings in fracture mechanics. The displacement field in an infinite body was expressed in terms displacement discontinuity for a growing plane crack with arbitrary shape and orientation. The complete field response, including far-field, near-field, and intermediate-field motion has been studied. The displacement field in an infinite body was converted into the normal surface motion at the sensor site at the boundary of a half-space through local modification, considering the first reflection and mode conversion of the incident P and S waves at the traction-free boundary. The surface motion obtained is valid for finite crack propagation, while an algebraic equation for microcracking is given as a special case for illustration. Using these results, we will be able to analyze the fracture mode and relative location and orientation of sensor and crack plane according to the waveform obtained.

Laser-speckle angular-displacement sensor: theoretical and experimental study

A novel, to our knowledge, method for the measurement of angular displacement for arbitrarily shaped objects is presented in which the angular displacement is perpendicular to the optical axis. The method is based on Fourier-transforming the scattered field from a single laser beam that illuminates the target. The angular distribution of the light field at the target is linearly mapped on a linear image sensor placed in the Fourier plane. Measuring this displacement facilitates the determination of the angular displacement of the target. It is demonstrated both theoretically and experimentally that the angular-displacement sensor is insensitive to object shape and target distance if the linear image sensor is placed in the Fourier plane. A straightforward procedure for positioning the image sensor in the Fourier plane is presented. Any transverse or longitudinal movement of the target will give rise to partial speckle decorrelation, but it will not affect the angular measurement. Furthermore, any change in the illuminating wavelength will not affect the angular measurements. Theoretically and experimentally it is shown that the method has a resolution of 0.3 mdeg ( approximately 5 murad) for small angular displacements, and methods for further improvement in resolution is discussed. No special surface treatment is required for surfaces giving rise to fully developed speckle. The effect of partially developed speckle is considered both theoretically and experimentally.